Nox reduction system utilizing pulsed hydrocarbon injection

ABSTRACT

Hydrocarbon co-reductants, such as diesel fuel, are added by pulsed injection to internal combustion engine exhaust to reduce exhaust NO x  to N 2  in the presence of a catalyst. Exhaust NO x  reduction of at least 50% in the emissions is achieved with the addition of less than 5% fuel as a source of the hydrocarbon co-reductants. By means of pulsing the hydrocarbon flow, the amount of pulsed hydrocarbon vapor (itself a pollutant) can be minimized relative to the amount of NO x  species removed.

RELATED APPLICATIONS

This Application is a continuation-in-part of U.S. patent applicationSer. No. 09/295,006, filed Apr. 20, 1999, entitled “NITROGEN OXIDEREMOVAL USING DIESEL FUEL AND A CATALYST.” The application isincorporated herein by reference.

The United States Government has rights in this invention pursuant toContract No. W-7405-ENG-48 between the United States Department ofEnergy and the University of California for the operation of LawrenceLivermore National Laboratory.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the reduction of NO_(x) from engineexhaust emissions, and more particularly to the removal of NO_(x) fromdiesel engine exhaust.

2. Description of Related Art

The control of NO_(x) emissions from vehicles is a worldwideenvironmental problem. Gasoline engine vehicles can use newly developedthree-way catalysts to control such emissions, because their exhaustgases lack oxygen. But so-called “lean-burn” gas engines, and dieselengines too, have so much oxygen in their exhausts that conventionalcatalytic systems are effectively disabled. Lean-burn, high air-to-fuelratio, engines are certain to become more important in meeting themandated fuel economy requirements of next-generation vehicles. Fueleconomy is improved since operating an engine stoichiometrically leanimproves the combustion efficiency and power output. But excessiveoxygen in lean-burn engine exhausts can inhibit NO_(x) removal inconventional three-way catalytic converters. An effective and durablecatalyst for controlling NO_(x) emissions under net oxidizing conditionsis also critical for diesel engines.

According to a report published February 1992 by the U.S. EnvironmentalProtection Agency, (Office of Air and Radiation, Office of Air QualityPlanning and Standards, Research Triangle Park, N.C. 27711), there are,in general, four approaches to controlling NO_(x) emissions fromcombustion sources. For example, controlling NO_(x) formation bymodifying the combustion operating conditions, by modifying thecombustion equipment, by fuel switching, and by post combustion controlof NO_(x) by flue or exhaust gas treatment. The first three approachesreduce the original formation of NO_(x). The latter converts the NO_(x)that was formed (in the exhaust gas) to something more benign.

With respect to lean-burn engines, catalysts (i.e., catalysts that candecompose NO_(x) to N₂ and O₂ in oxygen-rich environments) that have theactivity, durability, and temperature window required to effectivelyremove NO_(x) from the exhaust have not been effective. Prior artlean-NO_(x) catalysts are hydrothermally unstable. A noticeable loss ofactivity occurs after relatively little use, and even such catalystsonly operate over very limited temperature ranges. Conventionalcatalysts are therefore inadequate for lean-burn operation and ordinarydriving conditions. An alternative is to use catalysts that selectivelyreduce NO_(x) in the presence of a co-reductant, e.g., selectivecatalytic reduction (SCR) using ammonia as a co-reductant.

Using co-existing hydrocarbons in the exhaust of mobile lean-burngasoline and diesel engines as a co-reductant is a more practical,cost-effective, and environmentally sound approach. The search foreffective and durable SCR catalysts that work with hydrocarbonco-reductants in oxygen-rich environments is a high-priority issue inemissions control and the subject of intense investigations byautomobile and catalyst companies, and universities, throughout theworld.

SCR catalysts that selectively promote the reduction of NO_(x) underoxygen-rich conditions in the presence of co-reductant hydrocarbons areknown as lean-NO_(x) catalysts. More than fifty such SCR catalysts areconventionally known to exist. These include a wide assortment ofcatalysts, reductants, and conditions. The relatively expensive noblemetal catalysts have exhibited high activity. Unfortunately, justsolving the problem of catalyst activity in an oxygen-rich environmentis not enough for practical applications. Like most heterogeneouscatalytic processes, the SCR process is susceptible to chemical and/orthermal deactivation. The excess oxygen adsorbs preferentially on thenoble, precious metal, e.g., Pt, Rh, and Pd, surfaces in the catalyst,and inhibits a chemical reduction of NO_(x) to N₂—instead promoting theoxidation of unburned hydrocarbons and carbon monoxide. This is becausethe CO and hydrocarbon reductants tend to react more quickly with thefree oxygen, O₂, present in the exhaust gas than the oxygen associatedwith nitrogen in NO_(x). Also, many lean-NO_(x) catalysts are toosusceptible to water vapor and high temperatures. As an example, theCu-zeolite catalysts deactivate irreversibly if a certain temperature isexceeded. The deactivation is accelerated by the presence of water vaporin the stream. In addition, water vapor suppresses the NO reductionactivity even at lower temperatures.

Thus, the problems encountered in lean-NO_(x) catalysts include lessenedactivity of the catalyst in the presence of excessive amounts of oxygen(preference for oxidation of CO and hydrocarbons), reduced durability ofthe catalyst in the presence of water, sulfur, and high temperatureexposure, and narrow temperature windows in which the catalyst isactive. Practical lean-NO_(x) catalysts must overcome all three problemssimultaneously before they can be considered for commercial use.

Another major source of catalyst deactivation is high temperatureexposure. This is especially true in automobile catalysts wheretemperatures close to 1000° C. can exist. The high-temperatures attackboth the catalyst precious metal and the catalyst carrier, e.g., gammaalumina (γ-Al₂O₃). Three-way catalysts, for instance, are comprised ofabout 0.1 to 0.15 percent precious metals on a γ-Al₂O₃ wash coat, anduse La₂O₃ and/or BaO for a thermally-stable, high surface area γ-Al₂O₃.Even though the precious metals in prior art catalysts were initiallywell dispersed on the γ-Al₂O₃ carrier, they were subject to significantsintering when exposed to high temperatures. This problem, in turn, ledto the incorporation of certain rare earth oxides such as CeO₂ tominimize the sintering rates of such precious metals.

In one high temperature application described in US. Pat. No. 5,618,505,issued to Subramanian et al., researchers have attempted to reduceNO_(x) from internal combustion engine exhaust with relativelyinexpensive base-metal-containing lean-NO_(x) catalysts using a propanehydrocarbon coreductant. However, successful NO conversion percentagesabove 30 are only obtained with propane co-reductant at temperaturesexceeding 450° C. Such results are impractical for most, if not all,diesel internal combustion engine exhaust. Furthermore, tests of ninemodel fuels and a diesel fuel injected into an exhaust stream have shownno higher than 43% NO_(x) conversions. See Collier and Wedekind, TheEffect of Hydrocarbon Composition on Lean NO _(x) Catalysts, SAETechnical Series 97300, Int. Fall Fuels & Lub Meeting & Expos., Tulsa,Okla., (October 1997).

The challenge still exists for lean-NO_(x) catalysts promotion of NO_(x)reduction at the lower combustion temperatures associated with dieselexhaust. Modifications of existing catalyst oxidation technology aresuccessfully being used to address the problem of CO and hydrocarbonemissions, but no present solution exists for NO_(x).

Another existing challenge is to minimize the exhaust emission of theunused portion of added hydrocarbon vapor co-reductants duringlean-NO_(x) catalytic promotion of NO_(x) reduction. Added hydrocarbonvapor coreductants can be directly injected into an exhaust stream in acontrolled manner using flow controllers or vaporizing hydrocarbonliquids. Since utilization of the hydrocarbon vapor stream duringcatalytic reduction of NO_(x) is not 100% efficient, unused hydrocarbonvapors escape from present processing systems. A need exists to maximizethe reduction of NO_(x) to N₂ while minimizing the unused hydrocarbonvapor emission.

SUMMARY OF THE INVENTION

The present invention provides a method for catalytically reducingNO_(x) emissions, particularly emissions from diesel engine exhaust, byintermittently injecting additional hydrocarbons into an engine exhaust.The present invention also provides a vehicle with reduced NO_(x)emissions that utilizes such a method, particularly a vehicle having adiesel engine. The invention further provides a system for attachment toan engine with an oxygen-rich exhaust, particularly a diesel exhaust,for the reduction of NO_(x) emissions by the above method.

Briefly, the invention takes advantage of the discovery that underappropriate conditions for catalytic processes, the NO_(x) reductionreaction can occur in the absence of hydrocarbon vapor added to anengine exhaust. Thus, a continuous source of added hydrocarbon vapor tothe engine exhaust during catalytic NO_(x) reduction treatment isunnecessary. The present invention comprises treatment of an oxygen-richvehicle engine exhaust with a pulsed flow of added hydrocarbon vapor,preferably from a diesel fuel, in the presence of a catalyst, preferablycontaining an amphoteric catalyst support, such as one used in a SCRsystem, to enhance NO_(x) reduction. A NO_(x) reduction process of theinvention, conducted in the temperature range of diesel fuel combustion,i.e., from about 175 degrees C. to about 450 degrees C., results inconversion of at least 50% of exhaust NO_(x) to NO_(x) conversionproducts including N₂ and O₂, and the subsequent conversion of unuseddiesel fuel to produce benign exhaust products, such as CO₂.

An advantage of the present invention is that a method for NO_(x)emission reduction is provided that uses relatively small amounts ofadded hydrocarbons with inexpensive amphoteric catalytic components. Thereduction can allow the use of catalysts containing essentially nosupported metals for relatively inexpensive compliance to NO_(x)emission reduction laws.

Not only does the process improve the NO_(x) removal while utilizinginexpensive catalytic materials, but it also allows the combustion offuels with a concomitant reduction of at least 80% NO_(x), particularlyin an oxygen-rich vehicular diesel exhaust environment. Such anadvantage of the present invention is that a system is provided forreducing at least 50% of NO_(x) emissions with a fuel penalty of lessthan 5%. For instance, about 500 to about 3500 ppm of diesel fueladdition to a diesel engine exhaust in the presence of a SCR systempromotes well above 60% NO_(x) reduction in the temperature range below500 degrees C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a vehicle embodiment of the invention.

FIG. 2 is a flow chart of the method of the invention.

FIG. 3 is a cross sectional diagram representing a NO_(x) reduction unitof the invention.

FIG. 4 is a graph illustrating the percentage of NO_(x) reduction in adiesel engine exhaust by gamma alumina catalyst as a function of theconcentration of diesel fuel addition to the exhaust at 200 degrees C.,250 degrees C., and at 300 degrees C.

FIG. 5 is a graph of the removal of NO_(x) in an exhaust stream overtime in combination with a graph of the injected hydrocarbonconcentration into the exhaust stream.

FIG. 6 is a graph of the NO_(x) reduction response of a catalyticreductive system resulting from a pulsed flow of hydrocarbon vapor beinginjected into an engine exhaust stream.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a vehicle embodiment of the present invention, and isreferred to herein by the general reference numeral 10. The vehicle 10is provided with a fuel tank 12 that supplies an internal combustionengine 14 and a NO_(x) reduction unit 16. The fuel used may be #2 dieseloil and the engine 14 may be a diesel type common to busses and trucks.The engine 14 has an output of exhaust gas that is both rich in oxygenand oxides of nitrogen (NO_(x)), e.g., NO and NO₂. Oxygen-rich exhaustsare typical of diesel engines and lean-burn gasoline engines. SuchNO_(x) in the exhaust is environmentally undesirable. The exhaust and ahydrocarbon such as unused fuel from fuel tank 12, are input to theNO_(x) reduction unit 16 via exhaust outlet 14 a and fuel tank bleedline 15, respectively. Hydrocarbons in the fuel and a selectivecatalytic reduction (SCR) system are used in a one-step conversion ofhydrocarbons +NO_(x)→N₂, CO₂ and H₂O by the NO_(x) reduction unit 16. Amuffler 18 is used to quiet the otherwise noisy cleaned exhaust producedin NO_(x) reduction unit 16 via optional exhaust outlet 17. An oxidativesystem (not shown), which is usually catalytic, can be employed tooxidize and remove unused hydrocarbon (diesel fuel) from NO_(x)reduction unit 16 prior to final exhaust emission from the vehicle.

FIG. 2 illustrates a method embodiment (shown generally as 30) of thepresent invention for NO_(x) removal in oxygen-rich exhaust flows. TheNO_(x) reduction unit 16 of FIG. 1 represents an implementation ofmethod 30. A step 32 converts the NO_(x) in an oxygen-rich exhaust flowto N₂ by mixing hydrocarbon molecules (e.g., engine fuel) into theoxygen-rich exhaust flow and passing the (normally vaporous) mixturethrough or over a SCR catalyst, such as a relatively inexpensivegamma-alumina-containing catalyst. Although any conventional SCRcatalyst can be employed, catalysts having amphoteric supports,including all forms of gamma alumina, are preferred. Amphoterism isreferred herein in the classic sense, i.e., the reactivity of asubstance with both acids and bases, acting as an acid in the presenceof a base and as a base in the presence of an acid. Among the amphotericmetal oxides that have been shown to be active for reducing NO_(x) inthe presence of both oxygen and hydrocarbon in addition to Al₂O₃, areGa₂O₃ and ZrO₂. It is highly preferred that the SCR catalyst, i.e., aNO_(x) reducing catalyst, contain essentially no supported metalsdeposited onto the porous supports; however, if such supported metalsare employed, it is particularly preferred that such metals be arelatively inexpensive, non-noble metal such as Cu, Ni, Sn and the like,rather than expensive Pt, Pd or Rh.

Furthermore, complex hydrocarbons and mixtures of hydrocarbons, such asdiesel oil, can optionally be reduced to simpler hydrocarbon moleculesby cracking the complex hydrocarbon molecules with a plasma processor orother cracking means. In a subsequent step 34, an oxidizing catalyst,typically any conventional oxidizing catalyst, is used to convert theunused hydrocarbons and O₂ to more benign products such as CO₂.

Alternatively, a simple hydrocarbon may be supplied to the NO_(x)reduction unit 16. Some hydrocarbons may be better reductants or betterNO_(x) to N₂ promoters. A disadvantage of such an embodiment is that twodifferent supplies of hydrocarbons must be maintained aboard the vehicle10. An advantage of a preferred embodiment of the present invention isthat fuels, such as No. 1 or 2 diesel fuels, can serve as co-reductantswith a SCR catalyst to reduce NO_(x) and concurrently provide fuel forthe upstream exhaust-generating engine. Thus, only one uncombustedsource of hydrocarbons can be maintained aboard the vehicle.Nevertheless, other hydrocarbons which may be used, at least in part, asa co-reductant with the SCR catalyst include kerosene, propane, crackedNo. 1 diesel oil, and cracked No. 2 diesel oil. Since a preferredembodiment of the invention relates to NO_(x) reduction in industrialdiesel fuel-burning engines, stationary or in vehicles, where dieselfuel No. 2 is combusted, a highly preferred hydrocarbon co-reductantadded to the NO_(x)-polluted exhaust is No. 2 diesel fuel.

FIG. 3 illustrates a NO_(x) reduction unit (shown generally as 50) ofthe present invention. The NO_(x) reduction unit 50 is similar to theNO_(x) reduction unit 16 of FIG. 1 and similar in operation to theNO_(x) reduction method 30 of FIG. 2. The NO_(x) reduction unit 50comprises a cylindrical housing 52 with an atomized hydrocarbon inlet54, an engine exhaust inlet 56 and a processed exhaust outlet 58. Thehousing 52 need not be cylindrical and can take the form of an exhaustmanifold attached to an engine. The higher temperatures afforded byclose proximity of the NO_(x) reduction unit 50 to the engine arepreferred.

The exhaust and hydrocarbons are mixed in a chamber 66 between aninsulative bulkhead 72, which separates inlets 54 and 56, and insulativebulkhead 80 on which a catalytic converter 78 is mounted. The exhaustand hydrocarbon parameters may be made independently variable andmicrocomputer controlled to accommodate a variety of exhaust flow ratesbeing processed. Another parameter—temperature—is a feature of theinvention. The mixture of added hydrocarbons (particularly additivediesel fuel) is passed over or through catalytic converter 78 attemperatures normally less than 500 degrees C., more particularly lessthan 450 degrees C. and preferably in the range from about 175 degreesC. to about 425 degrees C., normally within the boiling temperatures ofdiesel fuel. In a preferred embodiment, hydrocarbons in a concentrationabove about 1000 ppm of the exhaust are added to the exhaust and passedover a SCR catalyst at a temperature above about 200 degrees C.

Optionally, a preprocessor 70 is constructed as a concentric metal tubethat pierces the bulkhead 72. The preprocessor 70 can crack the complexhydrocarbons provided from the inlet 54 into simpler hydrocarbons using,for instance, a non-thermal plasma. Furthermore, both the hydrocarbonsand a non-thermal plasma from a plasma converter (not shown) can bemixed in chamber 66 and used to convert NO in the flow from the engineexhaust inlet 56 into NO₂. Optionally, porous bulkhead 64 can bepositioned within chamber 66 to concentrate NO₂ with the hydrocarbons inthe area of the catalyst surface of catalytic converter 78.

However, in the principal thrust of the invention, catalytic converter78, mounted on bulkhead 80, provides for the selective catalyticreduction of the exhaust NO_(x) (predominantly NO₂ if the NO_(x)reduction is plasma-assisted or otherwise) to more environmentallybenign molecules, such as N₂, O₂, CO₂ and H₂O, using the addedhydrocarbon co-reductant mixed with the exhaust in chamber 66. Ingeneral, catalysts having an amphoteric support are utilized in theinvention; however, any SCR catalyst, i.e., lean-NO_(x) catalyst, can beemployed in the catalytic converter. The catalytic converter 78 maypreferably be configured as a bed of gamma alumina pellets, e.g.,γ-Al₂O₃. The catalytic converter 78 may also be configured as a washcoat of gamma alumina on a substrate.

An oxidative system, usually an oxidation catalyst 82 can be mounted ona bulkhead 84 and provides for the burning of any excess (unused)hydrocarbons not consumed by the catalytic converter 78. Preferably, theflow of hydrocarbons into the inlet 54 is controlled to minimize suchexcess hydrocarbons that must be burned by the oxidation catalyst 82.

Oxygen enhances the selective catalytic reduction of NO by hydrocarbons.Although not bound by any one theory, empirical evidence suggests thatthe NO_(x) reduction by lean-NO_(x) catalysts activate the NO byconverting it to NO₂, either in the gas phase or on the surface. The NO₂can then be reduced on the catalyst surface when in the presence ofhydrocarbons. The gas-phase formation of NO₂ is, in theory, probablysufficient to account for the observed rate of NO_(x) reduction byγ-Al₂O₃. Whether the heterogeneous oxidation of NO also takes place isnot clear. The functions of the active sites are complicated because amultitude of reactions happen on the surface. There are sites on whichthe NO may be activated by oxidation to NO₂, sites where the hydrocarbonmolecule is activated, sites where the carbon oxides are formed, andsites where the coupling of nitrogen-containing molecules take place.Individual sites may be involved in more than one step, or there may betwo or more different sites in close proximity acting as amulti-functional catalyst.

Catalysts that are active in selective catalytic reduction of NO byhydrocarbons usually have surface acidity, e.g., they possess surfacehydroxyl groups. The simplest surface on which selective catalyticreduction by hydrocarbons is observed is the amorphous, acidic form ofalumina, known as γ-Al₂O₃. In addition to having the best physicalsurface structure, e.g. surface area of 100-200 square meters per gram,γ-Al₂O₃ is also the most acidic form of stable alumina.

In FIG. 4, at incoming exhaust temperatures of 200 degrees C., 250degrees C., and 300 degrees C. to a catalytic converter, the percentageof total NO_(x) reduction in an exhaust from a diesel engine is comparedwith a varying diesel fuel concentration, using an additional unburnedportion of the diesel fuel burned in the diesel engine as theco-reductant. The catalyst contains particulate alumina, such as pelletsof pure γ-Al₂O₃. The NO_(x) reduction is attributed the combination ofadditive diesel fuel concentration and the activity of the catalyst. Theconcentrations of NO and NO₂ (NO_(x)) are detected and quantified byboth chemiluminescence and infrared absorbance. The NO_(x) reduction ispresumably due to increased N₂, since the amount of N₂O and any otheroxides of nitrogen, like HONO₂, is negligible compared to the reductionin NO_(x) concentration. The maximum NO_(x) reduction shown in FIG. 4can be increased by increasing the amount of additive diesel fuel,increasing the γ-Al₂O₃ and/or modifying the exhaust gas flow rate.

In the three experiments (data summarized in FIG. 4) that are conductedin view of the scheme of FIG. 3 (without options), the respectiveincoming engine-exhaust gas temperatures are about 200, 250, and 300°C., using actual diesel engine exhaust, which typically contain initial600 ppm NO_(x). About 500 ppm diesel fuel is initially injected in eachexperiment through inlet 54 to the NO_(x)-containing gas exhaust streaminleted through inlet 56 in chamber 66. After passing through the gammaalumina catalyst in catalytic converter 78, less than about 10% of theNO_(x) is reduced at the lower temperatures while slightly above 20% isreduced at the higher temperature. A total NO_(x) reduction of greaterthan about 50% is achieved after passing through the catalytic converterwhen the additive diesel fuel concentration is increased to within therange from about 1,300 ppm to about 1,600 ppm. The data exhibits anon-linear effect for the NO_(x) reduction at a given additive dieselfuel concentration for each inlet exhaust temperature.

Such a non-linear effect can be applied to diesel engine NO_(x)reduction control, particularly since the exhaust temperatures of theexperiments are within the range of typical industrial diesel exhausttemperatures and the additive diesel fuel concentrations indicategreater than 50% NO_(x) reduction with less than a 5% fuel penalty forthe overall diesel combustion system. For instance, at an exhausttemperature of 250 degrees C. about 1,600 ppm of additive diesel fuelprovides co-reductant activity with the gamma alumina catalyst theeffects greater than 50% NO_(x) reduction. A 1,600 ppm additive dieselfuel concentration is only about a 2.2%/fuel penalty. Furthermore, sucha NO_(x) reduction improvement from less than 10%. NO_(x) reduction withan additive diesel fuel concentration of about 1,100 ppm is clearlyunpredicted and unexpected. Accordingly, even at such a low exhausttemperature as 200 degrees C, the results illustrated in FIG. 4 clearlysuggest that relatively high percentages of NO_(x) reduction can beachieved at concentrations of over 3000 ppm additive diesel fuel, i.e.,still less than a 5%. fuel penalty.

In one embodiment illustrating the present invention in view of theblock diagram of FIG. 1, an engine exhaust 14 a initially containing 200ppm of NO_(x) is treated for NO_(x) reduction in the presence of an SCRcatalyst in NO_(x) reduction unit 16. An initial concentration of 4545ppm C1 of hydrocarbon vapor from a periodic pulse controller (i.e., agas flow controller adapted to periodically or intermittently injectgas) or the like is injected into NO_(x) reduction unit 16 from,through, or within, for example, fuel tank bleed line 15 to co-reduceNO_(x) to a level of about 165 ppm. Such a pulse of hydrocarbon vaporinjected for approximately 10 minutes is then stopped for 25 minutes;however, NO_(x) reduction continues to occur as the hydrocarbon vaporconcentration falls to less than 100 ppm in about the first 5 minutes ofstoppage. Although the NO_(x) reduction declines over about the entire25 minutes of stoppage from a removal of above about 165 ppm NO_(x) to aremoval of about 70 ppm NO_(x), nevertheless NO_(x) species continues tobe removed (reduced) from the exhaust stream even though the hydrocarbonlevel is essentially negligible (e.g., less than 50 ppm) over thestopped-pulse period (e.g., about 25 minutes). Such a treatmentindicates that NO_(x) removal can be achieved essentially in the absenceor minimus of added hydrocarbon vapor under suitable catalytic NO_(x)reduction conditions. FIG. 5 exhibits a summary of the data for such anembodiment for the single hydrocarbon pulse and single stop.

In all the vehicular, method and system embodiments described herein,repeated pulsing of the added (vapor) hydrocarbons to the engine exhaustcan restore the gradually declining NO_(x) reduction levels to a desiredconversion level. In general, the hydrocarbon injection andnon-injection (stopped) time intervals are predetermined to besufficient to maintain at least a desired percentage of NO_(x)reduction, such as at least 50%, or more. On a overall basis, the netamount of added hydrocarbon to the engine exhaust (vs. continuouslyinjected hydrocarbons) can be substantially decreased during the entireoperation of the vehicle, method, system, etc., for a desiredpredetermined level of NO_(x) reduction. FIG. 6 plots the response of acatalyst system with a pulsed flow of added hydrocarbon vapor to theexhaust flow. Over hydrocarbon injection intervals of about 10 minutes(i.e., 10 min. hydrocarbon injection followed by 10 min. no hydrocarbninjection followed by 10 min. hydrocarbon injection followed by 10 min.no hydrocarbon injection, etc.), NO_(x) reduction is shown to bemaintained in the range from above about 50 ppm to about 100 ppm duringthe overall operation, including all the pulsing cycles. The NO_(x)reduction continues when the hydrocarbon level falls, but is restored asthe hydrocarbon vapor is again injected into the catalytic system.

Preferably, a pulse of hydrocarbon into the engine exhaust can coincidewith the bursts of NO_(x) output from the engine. For instance, atypical diesel automobile engine can produce an initial large burst ofNO_(x) for about 200 seconds, followed by narrower spikes lasting forabout 400 seconds, and then a relatively constant NO_(x) emissionlasting about 200 seconds. Under such NO_(x) output engine conditions,typical intermittent hydrocarbon (diesel fuel) pulsing into the exhaustcan entail about 200 seconds of diesel fuel injection followed by 400seconds of stoppage followed by 200 seconds of diesel fuel injection.However, the remainder of the periods can require intermittentinjections and stoppages to match engine NO_(x) output and desiredreduction levels. In another instance involving diesel truck engineexhaust NO_(x) emissions, a NO_(x) burst for 50 seconds can be followedby a 100 second stoppage as well as other NO_(x) bursts lasting 300seconds with only a 50-second gap before the next burst, and so on.Optimization of the conditions under which the catalyst system isoperated by one of ordinary skill in the art provides a system thatremoves NO_(x) species at the desired rate while minimization of theaverage injected or added hydrocarbon vapor level is achieved.

Although particular embodiments of the present invention have beendescribed and illustrated, such is not intended to limit the invention.Modifications and changes will no doubt become apparent to those skilledin the art, and it is intended that the invention only be limited by thescope of the appended claims.

The invention claimed is:
 1. A method for reducing nitrogen oxides(NO_(x)) in oxygen-rich exhausts from high-temperature combustion, themethod comprising the steps of: intermittently adding hydrocarbons to anengine exhaust comprising NO_(x) to produce a hydrocarbon-added engineexhaust; and converting NO_(x) in the hydrocarbon-added engine exhaustwith a selective catalytic reduction (SCR) catalyst, to a gas flowincluding N₂ and O₂.
 2. The method defined in claim 1 wherein saidhydrocarbons boil in the range from about 150 degrees C. to about 450degrees C. and said converting occurs at a temperature less than about450 degrees C.
 3. The method of claim 1 wherein said hydrocarbon-addedengine exhaust contains sufficient hydrocarbons to result in anon-linear effect of an increased NO_(x) reduction percentage fromcontact with said SCR catalyst.
 4. The method of claim 1 wherein atleast a portion of said NO_(x) is converted to NO₂ in the presence of anon-thermal plasma.
 5. The method of claim 1 wherein at least 50% ofsaid NO_(x) is reduced.
 6. An apparatus comprising a catalytic converterand a diesel fuel inlet flow controller, said apparatus comprising: anengine-exhaust gas inlet; a diesel fuel inlet; a diesel fuel pulsecontroller connected to said diesel fuel inlet; and a reductive stageconvert of NO_(x) connected to receive a mixture of NO_(x) from theengine-exhaust gas inlet and diesel fuel from the diesel fuel inlet, theconvert comprising an amphoteric catalyst support that further serves toconvert NO_(x) to gases that include N₂, CO₂, and H₂O.
 7. The converterof claim 6, wherein: said catalyst of said reductive stage convertconsists essentially of a gamma-alumina catalyst (γ-Al₂O₃).
 8. Theconverter of claim 6 further comprising a plasma converter upstream ofsaid catalyst.
 9. A method for reducing NO_(x) contained in anoxygen-rich diesel engine exhaust, said method comprising: injecting adiesel fuel into said diesel engine exhaust for a predetermined timeinterval to produce a mixture containing said diesel engine exhaust andsaid diesel fuel, said diesel fuel in a concentration from about 500 ppmto about 3500 ppm; contacting a selective catalytic reduction (SCR)catalyst with said mixture at a temperature less than about 450 degreesC. to reduce said NO_(x) contained in said diesel engine exhaust; andstopping said injecting and continually contacting said diesel engineexhaust with said catalyst to reduce said NO_(x) contained in saiddiesel engine exhaust.
 10. The method of claim 9 wherein saidtemperature is in the range from about 175 degrees C. to about 425degrees C.
 11. The method of claim 10 wherein injecting said diesel fuelinto said exhaust in a concentration greater than 1000 ppm at atemperature above about 200 degrees C.
 12. The method of claim 11wherein said concentration of diesel fuel comprises less than 10% of adiesel fuel requirement to produce said diesel engine exhaust.
 13. Themethod of claim 9 wherein said SCR catalyst comprises gamma alumina. 14.The method of claim 9 wherein after contacting said mixture with saidSCR catalyst, an unconverted portion of said diesel fuel is subsequentlyoxidized to CO₂.
 15. A vehicle with reduced NO_(x) engine exhaustemissions, comprising: a fuel supply of diesel fuel; an internalcombustion engine connected to receive a major portion of said fuelsupply of diesel fuel and to propel a vehicle, and having an oxygen-richexhaust comprising NO_(x); a first reactor comprising a catalyst forNO_(x) reduction gas treatment connected to receive pulsed inlettedminor portions of said fuel supply of diesel fuel and further connectedto receive said oxygen-rich exhaust comprising NO_(x), and connected tooutput therefrom a product comprising N₂ that has been converted fromsaid NO_(x) and noncombusted hydrocarbons from said diesel fuel, and asecond reactor for collection and combustion of said noncombustedhydrocarbons connected to receive said product of the first reactor withsaid NO_(x) and connected to receive said noncombusted hydrocarbons, andoperably connected to output a second exhaust with reduced NO_(x)emissions.
 16. The vehicle of claim 15 wherein said first reactorcomprises said second reactor.
 17. The vehicle of claim 15 wherein saidfirst reactor comprises a selective catalytic reduction (SCR) catalystand said second reactor comprises an oxidizing catalyst.
 18. The vehicleof claim 15 wherein said first reactor is adapted to receive said minorportion of said fuel supply of diesel fuel in an amount less than 10% ofsaid fuel supply of diesel fuel.
 19. The vehicle of claim 18 whereinsaid minor portion of said fuel supply of diesel fuel comprises lessthan 5% of said fuel supply of diesel fuel.
 20. The method of claim 12wherein at least 80% of said NO_(x) is reduced.
 21. A method forreducing nitrogen oxides (NO_(x)) in oxygen-rich exhausts fromhigh-temperature combustion, the method comprising the steps of:intermittently adding diesel fuel boiling in the range from about 150degrees C. to about 450 degrees C. to an engine exhaust comprisingNO_(x) to produce a diesel fuel-added engine exhaust; converting NO_(x)in the diesel fuel-added engine exhaust at a temperature less than about450 degrees C., with a selective catalytic reduction (SCR) catalyst, toa gas flow including N₂ and O₂; and converting NO_(x) in the engineexhaust in the absence of added diesel fuel at a temperature less thanabout 450 degrees C., with a selective catalytic reduction (SCR)catalyst, to a gas flow including N₂ and O₂.
 22. The method of claim 21wherein said gas flow comprises an unconverted portion of said dieselfuel and said unconverted portion is subsequently oxidized to CO₂.